I don't know if it is normal, but the Adcom 555s that I got for my son years ago both exhibited the same behavior. Didn't seem to hurt the speakers.
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My GFA-535 Adcoms also did this on shutdown.Make sure to keep the volume low when you power down(I don't know if this helps,but it sure can't hurt!!).I think it's capacitors discharging that causes this.I might be wrong,have been before!!I asked Adcom about this thru their website but never got an answer.I guess they don't care about people with older equipment.
I've had them do this it during shutdown on other model Adcom amps I own.I don't remember if this was supposed to be the normal,but mine sure did it.They are in a closet somewhere.I just found it on a Adcom site,and they say the same as Tpreaves said.I thought it may be the caps too but wasn't sure until now.Adcom>>[http://www.adcom.com/faqs.aspx#faq10]